1. Field of the Invention
The present invention relates to a device and system for waveform shaping.
2. Description of the Related Art
A Mach-Zehnder interferometer (MZI) type optical gate is known as a conventional waveform shaping device for performing waveform shaping on the optical level. This optical gate is configured by integrating a Mach-Zehnder interferometer including first and second nonlinear optical media each for providing a phase shift on an optical waveguide substrate, for example. Probe light as continuous wave (CW) light is equally divided into two components, which are in turn supplied to the first and second nonlinear optical media. The optical path length of the interferometer is set so that output light is not obtained by interference of the two components of the probe light.
An optical signal is further supplied to one of the first and second nonlinear optical media. By properly setting the powers of the optical signal and the probe light, a converted optical signal synchronous with the optical signal is output from the optical gate. The converted optical signal has the same wavelength as that of the probe light.
It has been proposed to use a semiconductor optical amplifier (SOA) as each of the first and second nonlinear optical media. For example, an InGaAs-SOA having opposite end faces treated with antireflection coatings is used as each nonlinear optical medium in a 1.5 xcexcm band, and these nonlinear optical media are integrated on an InP/GaInAsP substrate to fabricate an optical gate.
A nonlinear optical loop mirror (NOLM) is known as another conventional waveform shaping device. The NOLM includes a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path for connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path.
By forming a part or the whole of the loop optical path from a nonlinear optical medium and supplying probe light and an optical signal respectively to the first optical path and the third optical path, a converted optical signal is output from the second optical path.
An optical fiber is generally used as the nonlinear optical medium in the NOLM. In particular, a NOLM using a SOA as the nonlinear optical medium is referred to as an SLALOM (Semiconductor Laser Amplifier in a Loop Mirror).
In each conventional waveform shaping device mentioned above, the converted optical signal is generated by a nonlinear effect based on the optical signal and the probe light supplied. However, since there is a limit that the wavelength of the converted optical signal generated is the same as the wavelength of the probe light supplied, the degree of freedom of wavelength conversion is small in the case of performing waveform shaping or obtaining an optical gate function.
This application relates to U.S. application Ser. Nos. 09/217,018, 09/560,723 and 09/571,384, and which are incorporated herein by reference.
It is therefore an object of the present invention to provide a device for waveform shaping which can increase the degree of freedom of wavelength conversion.
It is another object of the present invention to provide a novel system including such a device. Other objects of the present invention will become apparent from the following description.
In accordance with a first aspect of the present invention, there is provided a device comprising first and second nonlinear loop mirrors. Each of the first and second nonlinear loop mirrors comprises a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path formed of a nonlinear optical medium for connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path. The second optical path of the first nonlinear loop mirror is optically connected to the third optical path of the second nonlinear loop mirror.
This device operates in the following manner, for example. First probe light having a first wavelength is supplied to the first optical path of the first nonlinear loop mirror. An input optical signal having a second wavelength different from the first wavelength is supplied to the third optical path of the first nonlinear loop mirror. An intermediate optical signal having the first wavelength and synchronous with the input optical signal is supplied from the second optical path of the first nonlinear loop mirror to the third optical path of the second nonlinear loop mirror. Second probe light having a third wavelength different from the first wavelength is supplied to the first optical path of the second nonlinear loop mirror. As a result, an output optical signal having the third wavelength and synchronous with the intermediate optical signal (i.e., the input optical signal) is output from the second optical path of the second nonlinear loop mirror.
The sign of the difference between the first wavelength and the second wavelength may be made equal to the sign of the difference between the third wavelength and the first wavelength. In this case, it is possible to increase a wavelength difference between the input optical signal supplied to the first nonlinear loop mirror and the output optical signal output from the second nonlinear loop mirror.
Further, the third wavelength may be made substantially equal to the second wavelength. In this case, it is possible to perform the conversion from the optical signal supplied to the first nonlinear loop mirror to the optical signal output from the second nonlinear loop mirror without the generation of a wavelength difference between these optical signals.
Thus according to the present invention, it is possible to provide a device for waveform shaping which can increase the degree of freedom of wavelength conversion. Furthermore, it is also possible to obtain an additional effect such that more effective waveform shaping can be performed over the prior art in accordance with the principle to be hereinafter described.
At least one of the first and second probe lights may comprise clock pulses. In this case, retiming on the optical level according to the clock pulses is allowed.
In accordance with a second aspect of the present invention, there is provided a device comprising N (N is an integer greater than 2) nonlinear loop mirrors cascaded. Each of the nonlinear loop mirrors, comprises a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path formed of a nonlinear optical medium for connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path. The second optical path of the i-th (i is an integer satisfying 1xe2x89xa6ixe2x89xa6(Nxe2x88x921)) nonlinear loop mirror is optically connected to the third optical path of the (i+1)-th nonlinear loop mirror.
In accordance with a third aspect of the present invention, there is provided a system comprising a waveform shaping device provided by the device in accordance with the first aspect of the present invention, first and second probe light sources, and a first optical fiber transmission line. The first probe light source supplies first probe light having a first wavelength to the first optical path of the first nonlinear loop mirror. The first optical fiber transmission line supplies an input optical signal having a second wavelength different from the first wavelength to the third optical path of the first nonlinear loop mirror. The second probe light source supplies second probe light having a third wavelength different from the first wavelength to the first optical path of the second nonlinear loop mirror.
With this configuration, the waveform shaping in accordance with the present invention can be performed in an optical receiver provided at an output end of the first optical fiber transmission line.
This system may further comprise a second optical fiber transmission line optically connected to the second optical path of the second nonlinear loop mirror for transmitting the output optical signal. In this case, the waveform shaping in accordance with the present invention can be performed in an optical repeater provided between an output end of the first optical fiber transmission line and an input end of the second optical fiber transmission line.
In accordance with a fourth aspect of the present invention, there is provided a device comprising an optical branch for branching input signal light into first and second input signal lights, a clock regenerator for generating clock pulses according to the first input signal light, and a waveform shaping device for performing waveform shaping according to the second input signal light and the clock pulses. The waveform shaping device may be provided by the device according to the present invention.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.